how to construct django Q object matching none

Question

I wonder which is the correct way to construct a Q(...) object which matches no object in the queryset. It seems that both Q() and ~Q() match all objects!

Bijouterie

Which is the query you want to do? – Favien

I want to find a q such that M.objects.filter(q) is the same as M.objects.none() – Bijouterie

Why not using EmptyQuerySet? – Favien

Because the filter q is constructed once and used many times. I don't want to put an if...else each time it is used. – Bijouterie

Answer 1

Q(pk__in=[]) should do the trick.

Rodriquez

It's nice to note that the optimizer will correctly simplify complex expressions. Queries for ( Q(pk__in=[]) & Q(foo="bar") ) | Q(hello="world") will simplify the condition to WHERE "hello" = world. It also works with tilde ~ negations. Q(pk=None) does not get optimized the same way (presumably because pk can be overridden). – Irizarry

Answer 2

Q(pk=None) works fine.

Q(pk__in=[]) works fine as well and doesn't hit the database.

Thessalonian